Single wafer processing machines are increasingly being used in semiconductor fabrication facilities. One reason for this is to maximize control over the processing conditions. Processing conditions are often constituted by the temperature, intensity of radiant energy flux (e.g. ultraviolet or infrared), and/or the flux of atomic or molecular species (e.g. etchant)impinging the wafer. Another justification for using single wafer processing machines is to increase the uniformity of the processing conditions over the surface of the wafer. This is necessary so that each die produced from the wafer will have similar acceptable electrical performance characteristics, regardless of the part of the wafer from which it was cut. In this regard, it should be noted that holding the process conditions uniform over the entire wafer becomes more difficult as the wafer size increases according to the continuing trend in the semiconductor industry.
Certain processes require simultaneous application of radiant energy and reacting species to the wafer. For example, in conducting plasma ashing to remove used photoresist, heating radiation (e.g. infrared and visible light) is applied to the wafer through a window in the chamber and at the same time oxygen which has been excited by flowing it through a microwave plasma discharge is passed over the wafer. In this process the oxygen in an excited state reacts with the heated photoresist borne on the wafer, thereby removing it by oxidizing it into gaseous (e.g. CO.sub.2 and H.sub.2 O), and volatile low molecular weight products which are exhausted from the chamber through an exhaust orifice. Thus, in this process there is a need to achieve uniform heating radiation intensity over the wafer and uniform flux of excited oxygen reactants over the wafer.
FIG. 1 shows a schematic representation of a prior art plasma asher such as manufactured by Fusion Systems Corporation of Rockville, Md., the assignee of the instant invention. Referring to FIG. 1, gas is supplied from gas supply 1, and flows in conduit 4, through microwave exciter 2. The microwave exciter 2, as known in the art, may take the form of a microwave cavity with a gas conduit 4 passing through it. The gas is formed into a plasma as it flows in to the exciter. Microwave power from microwave generator 3 is fed (e.g. by a waveguide or by direct magnetron antenna coupling) into the microwave exciter to power the plasma. From the exciter the gas, which by action of the plasma excitation, has been chemically activated, flows into the wafer processing chamber, 5. The processing chamber 5, comprises upper, 6a and lower 6b inlet baffle plates, which serve to spread the flow of the reactive gas in order to make it more uniform. The baffle plates 6a, 6b have a symmetric arrangement of orifices as known in the art. Downstream, and below the lower inlet baffle plate 6b, the wafer 7 undergoing processing is positioned horizontally. The wafer is supported by three quartz standoffs 8 (two shown) above the lower wall 9 of the processing chamber. The lower wall, 9 also serves as a radiant energy window through which radiant heating power from incandescent bulbs 10a, 10b passes. Although only two bulbs 10a, 10b are shown in this schematic depiction, in actuality several arranged in a circle are used. The lower wall, 9 may be made of quartz glass and must be of sufficient thickness to bear the external ambient pressure when the chamber is operated under vacuum. The lower chamber wall 9 comprises a central orifice 9a. Upper exhaust pipe section 11a is also made of quartz, and is fused to the central orifice 9a of the lower chamber wall 9. Lower exhaust pipe section 11b is connected to a vacuum pump system 12. The central location of central orifice 9a is important because it contributes, along with arrangement of the baffle plates 6a, 6b to establishing a symmetrical flow of process gas in the processing chamber 5.
However the central location of the central orifice 9a, which requires the central location of upper exhaust pipe section 11a, causes difficulties in achieving uniform radiant power distribution over the wafer 7. The location of the upper exhaust pipe section 11a, interferes with the placement of radiant power sources and optics, and thereby prevents uniform distribution of radiant power over the surface of the wafer. Two examples which represent two known ways of achieving uniform irradiance over a plane, in this case the wafer, which are precluded by the presence of upper exhaust pipe section, 11a in the above described prior art processor, but which can be used according to the invention, as will be disclosed below, are the multiple variable control, multizone planar array illuminator approach, hereinafter referred to simply as "planar array", and the approach of using a custom designed surface of revolution reflector in combination with a single high powered source.
The planar array approach is precluded in the prior art processing chamber described above because it is a essential that the central lamp in the array be located directly below the center of the wafer. This cannot be arranged because the upper section 11a of the exhaust pipe passes through this location.
The surface of revolution reflector approach is precluded because the single high powered source used in this approach must be located directly below the center of the wafer, and as in the former planar array case this can not be arranged.